Duct burner with conical wire mesh and vanes

ABSTRACT

A recuperated microturbine system including a low temperature duct burner in the exhaust gas which accommodates significant variations in the exhaust gas flow from the recuperator, which burns the added hydrocarbon fuel, for example natural gas, reasonably efficiently, without generating a significant amount of additional NO x . A wire mesh burner is used to burn the added fuel in part of the exhaust gas from the recuperator. The duct burner is operated either in a more or less radiant combustion mode, or in a blue flame mode. The remainder of the recuperator exhaust gas, which bypasses the afterburner, is heated mainly by radiant heat transfer from the wire mesh burner. The use of the wire mesh burner minimises additional NO x  formation. In the duct burner the dynamic pressure of the exhaust gas is used to overcome the inherent static pressure loss of the wire mesh burner. The fuel flow, and exhaust air flow, to the burner are both controlled to provide stable combustion, in relation to the heat content required in the exhaust gas.

FIELD OF THE INVENTION

This invention relates to a so-called microturbine power plant. Moreparticularly, this invention relates to a combination of a microturbineand a secondary fuel burner in the gas turbine exhaust gas. The gasturbine drives an electrical generator and can have the generator unitbuilt directly onto the turbine shaft. The secondary fuel burner is usedto improve the quantity and quality of the heat content of the exhaustgas to a point where it can be used as a heat source for a number ofpurposes, such as for an absorption chiller, for space heating, forsteam generation, and for water heating. As a consequence of theproperties of the secondary fuel burner this invention provides a meansto obtain useful heat recovery without any increase in NO_(x) formation.

BACKGROUND OF THE INVENTION

In the past, electricity for commercial and domestic use has beengenerated on a large scale. Originally, either hydroelectric generationor coal fired power stations were used; in more modern practice oil,natural gas, and atomic energy are all used as alternatives to coal toprovide the required heat. In recent times, a need has been identifiedfor a small power plant to provide the electrical needs of a smallfacility, typically having a power requirement in the range of 25kW to500kW. For a power plant of this size, both atomic energy and coal arenot suitable as fuels. Small generating systems have been developedhaving this power rating in which the prime mover is a gas turbine, inwhich a liquid or gaseous hydrocarbon fuel is burnt to generate therequired heat. A typical microturbine unit of this type is described byGjerde, in U.S. Pat. No. 3,457,902. As described, the microturbinecombustor utilises only a small proportion of the oxygen in the intakeair to burn the hydrocarbon fuel; the microturbine exhaust gas typicallycontains about 16-19% oxygen by volume, compared to about 21% in theintake air. In order to minimise fuel use, the turbine exhaust gas isused to heat the intake air in a recuperator; such units are known asrecuperated microturbines.

In practise, it has been found that recuperated microturbine units havethree disadvantages.

First, although the exhaust gas leaving the recuperator is hot, it doesnot have enough heat content to be very useful beyond being used togenerate hot water. Second, although the combustion conditions can beimproved to use more of the oxygen to obtain higher exhaust gastemperatures, this will also cause increased NO_(x) formation.Recuperated turbines generally operate at a low rate of NO_(x) emission:values as low as 0.06 g/kWh or 1.8 ppm NO_(x) are known. Thiscombination of operating parameters provides exhaust gasses leaving therecuperator at a temperature of no more than 250° C., which is onlyuseful for space heating and hot water.

Third, in addition to Gjerde noted above, several proposals have beenmade to burn more fuel in at least a proportion of the exhaust gas, forinstance by Freeman et al. in U.S. Pat. No. 3,934,553 and by Vitterick,in U.S. Pat. No. 5,461,853. These proposals rely on the oxygenconsumption in the turbine combustor. Two difficulties then arise.First, the quantity of exhaust gas available from the turbine is onlyconstant if the working load on the turbine is also constant: inpractise this is not often the case. Second, if the added fuel is to beburnt efficiently the production of NO_(x) increases significantly.

There is still, therefore, a need for a recuperated microturbine unitwhich includes an afterburner system which will accommodate quitesignificant differences in the amount of available exhaust gas, whichwill increase the heat content of the exhaust gas, and which will burnthe added fuel at a low combustion temperature without any significantincrease in NO_(x) formation.

SUMMARY OF THE INVENTION

This invention seeks to provide a recuperated microturbine systemincluding a duct burner in the exhaust gas which accommodatessignificant variations in the exhaust gas flow from the recuperator,which burns the added hydrocarbon fuel, for example natural gas,reasonably efficiently, and which does not utilise combustion conditionswhich generate additional NO_(x). In the low temperature duct burner ofthis invention, a wire mesh burner is used to burn the added fuel inpart of the exhaust gas from the recuperator. This burner is operated ineither a more or less radiant combustion mode or preferably in theso-called “blue flame” mode, with lean gas mixtures at 50% of thestoichiometric fuel to gas ratio. The remainder of the recuperatorexhaust gas, which bypasses the afterburner, is heated by heat transferfrom the wire mesh burner. The fuel flow, and exhaust air flow, to theduct burner are both controlled to provide stable combustion, inrelation to the heat content required in the exhaust gas.

DETAILED DESCRIPTION OF THE INVENTION

Thus in a first broad embodiment this invention provides a duct burner,for use in an exhaust gas stream contained within an exhaust gas ductfrom a recuperated microturbine, including in combination:

(a) an exhaust gas intake tube, of the same cross sectional shape as,and smaller cross sectional area than, the exhaust gas duct, mountedsubstantially coaxially with the exhaust gas duct, which intake tube hasa first upstream end which receives exhaust gas and a second downstreamend;

(b) a throttle means located in the intake tube adjacent its firstupstream end constructed and arranged to control exhaust gas flow in theintake tube;

(c) a fuel feed for a hydrocarbon fuel located in the intake tubeadjacent to the throttle plate; and

(d) a wire mesh burner attached to the second downstream end of theintake tube and extending in a down stream direction from the second endof the intake tube;

wherein the wire mesh burner comprises a burner membrane consisting of anon-sintered fabric type membrane fabricated from heat resistantstainless steel fibre bundles in which the fibres have a substantiallyparallel arrangement and an equivalent fibre diameter of from about 1μto about 150μ.

Alternatively, the duct burner further includes:

(e) a perforated radiation tube, of the same cross section shape as, ofa larger cross sectional area than, and mounted substantially coaxiallywith, the intake tube to extend in a down stream direction from thesecond end of the intake tube to a point beyond the end of the wire meshburner.

Preferably, the wire mesh burner membrane has the same cross sectionalshape as the intake duct. More preferably, the wire mesh burner isconical, with its wide end attached to the second end of the intakeduct.

Preferably, the hydrocarbon fuel is natural gas.

Preferably, both the exhaust gas duct, the exhaust gas intake tube andthe perforated radiation tube if used are cylindrical, and the wire meshburner is conical, with its wide end attached to the second end of theintake duct. More preferably, both the exhaust gas duct, the exhaust gasintake tube and the perforated radiation tube if used are cylindrical,the radius of the exhaust gas intake tube is about one half of theradius of the exhaust gas duct, and the wire mesh burner is conical,with its wide end attached to the second end of the intake duct.

Alternatively, both the exhaust gas duct, the exhaust gas intake tubeand the perforated radiation tube if used have a rectangular crosssection, the cross sectional area of the exhaust gas intake tube isabout one quarter of the cross sectional area of the exhaust gas duct,and the wire mesh burner has a pyramidal shape, with its wide endattached to the second end of the intake duct.

In a further alternative, both the exhaust gas duct, the exhaust gasintake tube and the perforated radiation tube if used have a squarecross section, the cross sectional area of the exhaust gas intake tubeis about one quarter of the cross sectional area of the exhaust gasduct, and the wire mesh burner has a square pyramidal shape, with itswide end attached to the second end of the intake duct.

Preferably, sufficient hydrocarbon fuel is burnt in the wire mesh burnerto increase the temperature of the incoming exhaust gas by at leastabout 50° C. and by no more than about 500° C.

Preferably, the throttle means comprises a plate rotatable on a axiswhich is substantially perpendicular to the axis of the exhaust gasintake tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the attacheddrawings in which:

FIG. 1 shows a schematic layout for a typical microturbine powergenerating unit;

FIG. 2 shows schematically a burner assembly according to thisinvention, and

FIG. 3 shows the fuel inlet and throttle plate used in the lowtemperature duct burner shown in FIG. 2.

Referring first to FIG. 1, air is sucked into the microturbine unit 1through the air intake 2 by the compressor 3. The compressed air in line4 enters the recuperator 5, where it is heated further. The heatedcompressed air in line 6 passes into the combustor 7. In the combustor7, fuel entering in line 8 is mixed with the hot compressed air fromline 6, and some of the oxygen in it is burnt with the fuel. The hotcombustion gas passes through line 9 into the gas turbine 10. Exhaustgas leaves the turbine through line 11 to enter the recuperator 5, wheresome its heat content is recovered by heating the compressed airentering the recuperator in line 4. The cooled exhaust gas leaves therecuperator in the exhaust gas duct 12. The compressor 3 and the turbine10 are both mounted onto a common shaft 17, which also carries a highspeed electrical generator unit 18; rotation of the turbine 10 inducedby the gas flow in line 9 also rotates the generator 18 to provideelectrical power.

In the conventional recuperated microturbine unit, the exhaust gas isusually vented directly to a suitable stack 19, as indicated by thechain line 12A. In some units, the exhaust gas is passed through asecond heat recovery unit, for example to provide hot air for spaceheating, and then vented to a suitable stack.

In a recuperated microturbine unit according to this invention, theexhaust gas leaves the recuperator 5 in the exhaust duct 12 and passesto the duct burner unit 13. In the duct burner 13, more of the oxygen inthe exhaust gas in the exhaust gas duct 12 is burnt with further fuel inline 14, to raise the exhaust gas temperature. The heated exhaust gasleaves the burner in a continuation 15 of the exhaust gas duct. Afterpassing through the heat exchanger 16 the exhaust gas is vented througha suitable stack 19. A second fluid also flows through the heatexchanger 16, entering in line 20 and leaving in line 21. The secondfluid is usually either air, or water. When the second fluid is water,the heat exchanger 16 can be used to provide either hot water or toprovide steam.

Since the combustor 7 burns only some of the oxygen in the intake air inline 2, and since much of the heat in the exhaust gas in line 11 is usedin the recuperator 5 to heat the compressed air in line 4 ahead of thecombustor 7, the temperature of the exhaust gas in the exhaust gas duct12 is quite low: typical values are in the range of from about 150° C.to about 250° C. This value is too low to provide a useful amount ofheat. The exhaust gas at this point in the system will also usuallycontain at least about 17% oxygen.

In the duct burner, the temperature of the incoming exhaust gas can beincreased by at least about 100° C., and should not be increased by morethan about 500° C. This will raise the temperature of the exhaust gasfrom being in the range of from about 150° C. to about 250° C. to beingin the range of from about 600° C. to about 750° C. If the amount offuel burnt in the duct burner is increased to a level which willincrease the exhaust gas temperature to above about 750° C. thepossibility of increased NO_(x) formation increases significantly.

The duct burner 13 is shown in more detail in FIG. 2. Referring first toFIG. 2, the burner 13 is located within the exhaust gas duct 12 afterthe recuperator. Exhaust gas enters at the upstream end 30 in the duct12 and leaves at the downstream end 31 in the continuation 15 of theexhaust gas duct 12. The exhaust gas tube 32 is mounted coaxially withthe exhaust gas duct 12 by means of the vanes 33 at the upstream end andthe vanes 34 at the downstream end. These supports are convenientlylocated at an angle relative to the axis of the afterburner so as togenerate some swirling action within the exhaust gas, which improvesheat transfer to the exhaust gas. At the downstream end 36 of the tube32 a wire mesh burner 37 is mounted. The construction of this form ofburner is described in U.S. Pat No. 6,065,963, and the textiles used inits fabrication are described in U.S. Pat. No. 6,025,282. As shown aconical wire mesh burner is used, since the exhaust gas duct iscylindrical. It is preferred that the burner is of more or less the samecross sectional shape as the exhaust gas duct, and also that it shouldtaper inwardly toward the downstream end 31 of the duct 12. It thenfollows that since both square and rectangular ducts are known, for suchducts a square or rectangular wire mesh burner should be used. To obtainthe desired taper, the wire mesh burner will then be of a pyramidalshape, with a square, or rectangular, base.

The wire mesh burner 37 can be surrounded by a co-axial perforatedradiation tube 38, which extends sufficiently beyond the downstream endof the wire mesh burner 37 to contain the burner flame. The radiationtube is desirable when the duct burner is to be operated at a lowtemperature, in the so-called radiant heat mode. When the burner isoperated in the so-called blue flame mode a radiation tube 38 does notappear to be required.

As shown, the radiation tube 38 is conveniently attached to thedownstream vanes 34; it is also desirable to support the radiation tubewith a second set of vanes near to, or at, its downstream end, such asare shown at 34A. The radiation tube 38 is sized to provide a smallclearance 38A with the wire mesh burner 37. The perforations in theradiation tube are sized to optimise the radiant to convective heattransfer process from the burner to the remainder of the exhaust gasflowing in the space between the radiation tube 38 and the exhaust gasduct 12.

A combined throttle unit and fuel jet system 39 is mounted near theupstream end 35 of the tube 32; this is shown in more detail in FIG. 3.In FIG. 3 it is assumed that the hydrocarbon fuel is natural gas; theconstruction would be somewhat different for a liquid hydrocarbon fuel.The combined unit 39 is supported within the exhaust gas duct 12adjacent the vanes 33, so that the fuel feed line 14 can be locatedwithin one of the vanes. The line 14 is connected to a hollow spindle 40which includes several fuel jets 41. Both the number of jets, thelocation of the jets and the jet sizes, will be determined by the amountof fuel to be burnt. A throttle butterfly plate 42 is also mounted ontothe spindle 40. It is also contemplated that the fuel line 14 can beconnected to several fuel jets located in the wall of the tube 32adjacent the butterfly plate; a solid spindle 40 is then used. Thesetting of the throttle plate controls how much of the availableincoming exhaust gas flow passes through the tube 32, and equally howmuch passes through the annular space 32A between the tube 32 and theduct 12. When the duct burner is in use, the throttle plate 42 willusually be at an angle to the axis of the tube 32, and thus will alsoserve to generate some turbulence which assists in mixing the exhaustgas with the fuel. A suitable mechanism(not shown) is used to rotate thethrottle plate 42 on the spindle 40. The position of the throttle plate42 is chosen to fit the available gas flow and to provide an optimumfuel/oxygen mixture at the mesh burner. A spark ignition system, notshown, is used to ignite the gas on the burner.

In the arrangement shown, the throttle plate 42 rotates with the spindle40 so that the fuel jets 41 are often at an angle to the exhaust gasflow, which promotes mixing of the fuel and gas.

In operation, the fuel is burnt in a flame which is more or less in andon the wire mesh of the burner 37. The proportion of the incomingexhaust gas flowing through the tube 32 and the burner 37 is thus heateddirectly. The remainder of the exhaust gas flowing through the annularspace 32A is heated in part by radiation and by mixing with the gas fromthe burner 37 downstream of the end of the burner, or of the radiationtube 38 if this is present. The use of the co-axial tube 32 allows thedynamic pressure in the exhaust gas to overcome the static back pressureinherent in the wire mesh of the burner 37. This allows the burner tooperate without the need for additional fans or blowers in the gasstream.

What is claimed is:
 1. An afterburner, for use in an exhaust gas streamcontained within an exhaust gas duct from a recuperated microturbine,including in combination: (a) an exhaust gas intake tube, of the samecross sectional shape as, and smaller cross sectional area than, theexhaust gas duct, mounted substantially coaxially with the exhaust gasduct, which intake tube has a first upstream end which receives exhaustgas and a second downstream end; (b) a throttle means located in theintake tube adjacent its first upstream end constructed and arranged tocontrol exhaust gas flow in the intake tube; (c) a fuel feed for ahydrocarbon fuel located in the intake tube adjacent to the throttleplate; and (d) a wire mesh burner attached to the second downstream endof the intake tube and extending in a down stream direction from thesecond end of the intake tube; wherein the wire mesh burner comprises ashaped burner membrane consisting of a non-sintered fabric type membranefabricated from heat resistant stainless steel fibre bundles in whichthe fibres have a substantially parallel arrangement and an equivalentfibre diameter of from about 1μ to about 150μ.
 2. An afterburneraccording to claim 1 further including: (e) a perforated radiation tube,of the same cross section as, of a larger cross sectional area than, andmounted substantially coaxially with, the intake tube to extend in adown stream direction from the second end of the intake tube to a pointbeyond the end of the wire mesh burner.
 3. An afterburner according toclaim 1, wherein the wire mesh burner membrane has the same crosssectional shape as the intake duct.
 4. An afterburner according to claim1 wherein the hydrocarbon fuel is natural gas.
 5. An afterburneraccording to claim 1 wherein both the exhaust gas duct, the exhaust gasintake tube and the perforated radiation tube are cylindrical, and thewire mesh burner is conical, with its wide end attached to the secondend of the intake duct.
 6. An afterburner according to claim 5 whereinboth the exhaust gas duct, the exhaust gas intake tube and theperforated radiation tube are cylindrical, and a radius of the exhaustgas intake tube is about one half of a radius of the exhaust gas duct.7. An afterburner according to claim 1 wherein the throttle meanscomprises a plate rotatable on a axis which is substantiallyperpendicular to an axis of the exhaust gas intake tube.
 8. Anafterburner according to claim 5 wherein both the exhaust gas duct, theexhaust gas intake tube and the perforated radiation tube arecylindrical, a radius of the exhaust gas intake tube is about one halfof a radius of the exhaust gas duct, and the throttle means comprises acircular plate rotatable on a axis which is substantially perpendicularto an axis of the exhaust gas intake tube.
 9. An afterburner accordingto claim 1 wherein the fuel feed comprises a hollow spindle locatedtransversely across the intake tube having a plurality of fuel feedjets.
 10. An afterburner according to claim 1 wherein the fuel feedcomprises a hollow spindle located transversely across the intake tubeand the throttle means comprises a plate rotatable on the hollowspindle.